Many optically active aldehydes and ketones are valuable intermediates in the synthesis of relatively highly refined chiral substances of value and active ingredients, or are already themselves valuable fragrances and aroma substances.
EP-A 0 000 315 relates to a process for the preparation of optically active citronellal by hydrogenation of geranial or neral in the presence of a catalyst complex composed of rhodium and a chiral phosphine which is dissolved in the reaction system.
In J. Mol. Cat., 1982, volume 16, pages 51-59, T.-P. Dang et al. describe a process for the homogeneous-catalytic hydrogenation of α,β-unsaturated aldehydes, and the application of this process for the preparation of optically active citronellal. The catalysts used here were complex compounds of a rhodium carbonyl compound and a chiral diphosphine.
In Helv. Chim. Acta, 2001, volume 84, pages 230-242, footnote 4, Chapuis et al. also mention the asymmetric hydrogenation of geranial or neral to give optically active citronellal in the presence of a catalyst of Rh4(CO)12 and (R,R)-chiraphos (2R, 3R)-2,3-bis(diphenylphosphino)butane.
One problem when carrying out (homogeneous-catalytic) reactions catalyzed by means of soluble catalysts consists in the often inadequate stability of the catalyst complexes used or of the catalytically active metal or transition metal complex compound which is formed therefrom. Against the background of the often high price of such catalysts or catalyst precursors, homogeneous-catalytic reactions with complex transition metal catalysts can only be applied on an industrial scale in an economical manner in specific cases.
JP-A 52078812 describes a process for the hydrogenation of α,β-unsaturated aldehydes such as crotonaldehyde, cinnamaldehyde or α-methylcinnamaldehyde over homogeneous Rh catalysts under hydroformylation conditions in the presence of a triarylphosphine, a tertiary amine in stoichiometric amount and carbon monoxide.
WO 2006/040096 describes a process for the preparation of optically active carbonyl compounds by asymmetric hydrogenation of α,β-unsaturated carbonyl compounds with hydrogen in the presence of an optically active transition metal catalyst that is soluble in the reaction mixture and which has at least one carbon monoxide ligand and is characterized in that the catalyst is pretreated with a gas mixture comprising carbon monoxide and hydrogen and/or the asymmetric hydrogenation is carried out in the presence of carbon monoxide additionally introduced into the reaction mixture.
WO 2008/132057 likewise describes a process for the preparation of optically active carbonyl compounds by asymmetric hydrogenation of α,β-unsaturated carbonyl compounds which is based on the process disclosed in WO 2006/040096. To better control the carbon monoxide concentration in the reaction mixture during the hydrogenation, this process additionally includes the measures that the pretreatment of the catalyst precursor is carried out with a gas mixture comprising 20 to 90% by volume carbon monoxide, 10 to 80% by volume hydrogen and 0 to 5% by volume further gases, where the specified volume fractions add up to 100% by volume, at a pressure of 5 to 100 bar, excess carbon monoxide is separated off from the catalyst obtained in this way prior to use in the asymmetric hydrogenation, and the asymmetric hydrogenation is carried out in the presence of hydrogen with a carbon monoxide content of from 100 to 1200 ppm.
Particularly in the case of the last-mentioned processes for the asymmetric hydrogenation of α,β-unsaturated carbonyl compounds, it was possible to considerably improve the stability and/or the service life of the catalyst complexes used or of the catalytically active metal or transition metal complex compound which forms therefrom compared with conventional processes. On account of moderate and fluctuating catalyst activities, the said processes are in need of improvement as regards their conversion rates, particularly if they are carried out on an industrial scale.